1. Field of the Invention
The present invention relates to a device, a circuit and a module, more particularly to a light power compensation device, a light power compensation circuit and a detecting module.
2. Description of the Related Art
A forward bias voltage across a light-emitting diode (LED) may be influenced by environmental temperature. Referring to FIG. 1, each of three kinds of LEDs (blue LED, green LED and red LED) is driven by a constant 20 mA working current. When environmental temperature rises, the forward bias voltage of each of the LEDs drops, such that light power of each of the LEDs is reduced with rising environmental temperature. Therefore, simple utilization of a LED without performing power control thereon may result in a situation of unstable light power.
Referring to FIG. 2, a conventional light power compensation circuit 1 of an automatic power controller is disclosed in Taiwanese Patent No. I225190. The light power compensation circuit 1 is adapted to control light power of a LED 15 (or a laser diode) which acts as an optical head in an optical disc drive device. The light power compensation circuit 1 includes a sensor module 10, an integrator module 12, a signal source 11 and a driver module 13.
The sensor module 10 is for receiving light beams emitted from the LED 15 so as to detect light power thereof, and so as to generate a sensor voltage V3 which has a magnitude proportional to the light power of the LED 15. The light power satisfies: P=VF×I, in which P represents the light power of the LED 15, VF represents a forward bias voltage of the LED 15, and I represents a driving current of the LED 15. The sensor module 10 includes a photodetector 101 and a front-end amplifier 102. Detailed operations of the photodetector 101 and the front-end amplifier 102 are disclosed in Taiwanese Patent No. I225190.
The signal source 11 provides a reference voltage V1, and a value of the reference voltage V1 may be adjusted according to different anticipated light power.
The integrator module 12 is electrically coupled to the signal source 11 for receiving the reference voltage V1, is electrically coupled to the sensor module 10 for receiving the sensor voltage V3, and performs integration operation on a voltage difference between the reference voltage V1 and the sensor voltage V3 so as to obtain an integration voltage V2. When the light power decreases, the sensor voltage V3 decreases along with the light power such that the voltage difference increases and such that the integration voltage V2 increases along with the voltage difference. On the contrary, when the light power increases, the sensor voltage V3 increases along with the light power such that the voltage difference decreases and such that the integration voltage V2 decreases along with the voltage difference.
The driver module 13 is electrically coupled between the integrator module 12 and the LED 15. The driver module receives the integration voltage V2 from the integrator module 12, and outputs, according to the integration voltage V2, a driving current I proportional to the integration voltage V2 so as to drive the LED 15. The driver module 13 includes a gain-switchable amplifier 131 and a driving unit 132. Detailed operations of the gain-switchable amplifier 131 and the driving unit 132 are disclosed in Taiwanese Patent No. I225190.
When the forward bias voltage VF of the LED 15 drops with rising environmental temperature such that the light power of the LED 15 is reduced, the sensor voltage V3 generated by the sensor module 10 decreases accordingly. Furthermore, since the reference voltage V1 remains unchanged, the voltage difference V1-V3 between the reference voltage V1 and the sensor voltage V3 increases accordingly such that the integration voltage V2 and the driving current I correspondingly increase. Therefore, by increase of the driving current I for compensating decreased forward bias voltage VF, the light power P may remain fixed.
It is apparent from the foregoing that the conventional light power compensation circuit 1 adopts the photodetector 101 of the sensor module 10 for detecting a variation in light beams emitted from the LED 15 so as to obtain a variation in the light power of the LED 15. Subsequently, the conventional light power compensation circuit 1 adjusts the driving current I provided to the LED 15 according to a variation in the sensor voltage V3, such that an object that the light power of the LED 15 remains steady may be achieved. However, the conventional light power compensation circuit 1 has the following drawbacks:
Since directivity of the light beams emitted from the LED 15 is insufficient, positions of each of the photodetector 101 and the LED 15, a distance therebetween, ambient light interference and sensitivity of the photodetector 101 may affect the sensor voltage V3. Therefore, control of the light power may be inaccurate. Moreover, the sensor voltage V3 generated from the output of the photodetector 101 has different values for different wavelengths of the light beams emitted from the LED 15. Therefore, in view of the aforementioned reasons, the light power compensation circuit 1 which adopts the photodetector 101 may hardly keep the light power of the LED 15 steady when environmental temperature changes.